This invention relates generally to prosthetic joints and, more particularly, concerns a prosthesic joint for use as a hinge joint of the type wherein the loading forces of the joint are load-stabilized in use in the human body. This invention further relates to a prosthetic component for use in a prosthetic joint, and also to a connecting component for interconnecting two components of a prosthetic joint, especially of the load-stabilizing type.
Due to various causes, such as severe rheumatoid arthritis, for example in the metacarpophalangeal joint, it is often necessary to surgically replace the affected joint, including treatments such as resection arthroplasty. Such a surgical treatment not only relieves the discomfort and sufferings of the individual involved, but also is intended to provide a strong, stable joint which functions similarly to the natural joint which is being replaced. However, resection arthroplasty has had limited success in that no inherent stability is provided by the operation, but primarily relief of pain, and reliance is placed on the subsequent growth of soft tissues around the joint to provide any stability at all. By replacing a natural human joint with a mechanical prosthesis the individual may gain the virtual complete re-use of that joint if the prosthesis is appropriately designed.
In joints of the type wherein movement or hinge action occurs generally in one plane the commonly known prostheses which are offered for replacements have had a similar structure. For instance, the Steffee design (U.S. Pat. No. 3,506,982) for ginglymus joints employs a ball and socket for articulation. As commonly relied upon, the ball and socket configuration provides the hinging action for rotative movement with a sufficiently restrictive construction to confine the movement generally to one plane. In addition, the ball and socket design offers a construction whereupon the curved surface of the ball and the inside mating surface of the socket act as the load bearing surfaces of the joint during use, there being a substantially large available surface area over which to distribute the forces. Other similar devices are described in U.S. Pat. Nos. 3,694,821 and 3,760,427.
One of the deficiencies associated with the ball and socket construction of artificial joints is the lack of locking capacity to keep the components together. For example, in the Steffee design (noted above) the ball is snapped into the socket which has flexible lips surrounding the cavity to hold the ball. It is conceivable, and sometimes happens, that after this type of joint is implanted the components can become separated depending upon the forces which are exerted upon it. Such separation may occur because of the flexible lip structure of the socket and the general lack of any feasible and practical means to lock the ball in place in the implanted prosthesis. When dislocation of an endoprosthesis occurs it often requires another surgical operation to correct the problem.
Another deficiency in the ball and socket combination lies in the structure of the ball member itself. Generally the shank carrying the ball is slender and has a much smaller diameter than the ball it is carrying. Under severe loading forces of the joint, the ball transmits the forces along the shank for dissipation into the bone. In cases of unexpectedly large shear forces, if the ball does not become dislocated from the socket, the shank may fail and fracture thereby rendering the joint useless, thus requiring a surgical operation to either replace or repair the defective prosthesis.
Of course, it has also been known that one way to assure locking of the components of an endoprosthetic joint, while assuring rotative movement in one plane, is to pin the components together by means of an axle. Thus, instead of ball and socket articulation, the components of the prosthesis pivot around an axle thereby providing a hinged joint. Such a prosthetic joint is typified in U.S. Pat. No. 3,879,766. It has been found, however, that axle hinged joints, when cemented into the bone, generally become too rigid, thereby restricting motion and occasionally breaking; on the other hand, when such hinged joints are not cemented to the bone, the joint has occasionally produced localized areas of high stress which may cause erosion effects in the intramedullary canals. In addition, insertion of the axle, often from a side direction, to pin the two components together is not always convenient and straightforward in surgery. Depending upon the joint involved, and especially when a small joint such as one of the metacarpophalangeal finger joints is being replaced, there is very limited room in which to manipulate the axle, which produces an awkward operating procedure if such can be accomplished at all.
Another of the known prostheses which can be used as a replacement for a hinged joint is disclosed in U.S. Pat. No. 3,875,594. In that patent, a surgically implantable prosthetic joint is offered which is of a one-piece construction, molded of silicone rubber. While this type of joint replacement is flexible and eliminates the necessity of combining components together for an articulative joint, there are inherent shortcomings in this type of silicone rubber prosthesis. Primarily, there is a great reduction of strength when using silicone rubber instead of metallic components which are used in prostheses such as Steffee has disclosed. The reduced strength of the molded silicone rubber implant, in addition, limits the stability which the appropriate finger or limb is capable of subsequently developing.
As can be seen, the deficiencies in the various types of endoprosthetic devices as described above are indicative of the need for improvements in this field.